1. Field of the Invention
The present invention relates to a sensor used in combustion gas streams. In one embodiment, the sensor may be used to determine if non-equilibrium conditions or incomplete combustion are present. In another embodiment, the sensor may be used to determine the concentration of oxygen and/or unburnt fuel in the combustion gas stream.
2. Description of the Related Technology
Combustion of fossil fuels and hydrocarbon fuels is a widely used method for obtaining energy in a large number of industries. Complete combustion of hydrocarbon fuels converts the hydrocarbons in the fuel into carbon dioxide and water, in accordance with the reaction given in equation 1; EQU C.sub.x H.sub.y +(x+y/4)O.sub.2 .fwdarw.xCO.sub.2 +y/2H.sub.2 O1
In practice, there will also occur other incomplete combustion reactions, for example: EQU C.sub.x H.sub.y +(x/2)O.sub.2 .fwdarw.xCO+y/2H.sub.2 2
Incomplete combustion can occur if there is excess fuel. It can also occur with excess oxygen, if the fuel and air are not sufficiently well mixed.
Throughout this specification, the expression "combustion gas" is taken to mean off-gas, flue gas or exhaust gas from a combustion process. The combustion process may be the burning of any fossil-fuel or hydrocarbon fuel, and includes the burning of solid fuels, such as coal, for example as used in furnaces, kilns, boilers and the like, and the burning of liquid fuels, such as fuel oil and other petroleum products.
Sensors for measuring the oxygen content of combustion gases, such as engine exhaust gases and furnace off gases, have been described in the prior art. Such sensors generally utilise oxygen ion-conducting solid electrolytes. When opposed faces of the solid electrolyte are exposed to different oxygen partial pressures, an emf (E), or electrical potential difference, is developed which obeys the Nernst equation: ##EQU1## where: T is temperature (Kelvin)
R is the gas constant PA1 F is Faradays constant PA1 ln is the mathematical symbol for natural logarithm PA1 (pO.sub.2) inside is the oxygen partial pressure at the inside face of the electrolyte, and PA1 (pO.sub.2) outside is the oxygen partial pressure at the outside face of the electrolyte. PA1 a first electrode which, in use, is contacted by the combustion gas PA1 a second electrode PA1 a third electrode, PA1 each of said first, second and third electrodes being separated from another of said electrodes by solid electrolyte PA1 a first gas pathway means for passing a reference gas of known oxygen concentration to the third electrode PA1 a second gas pathway means for passing a combustion gas to the second electrode, the second gas pathway means including an oxidation catalyst arranged such that the combustion gas passing along the second gas pathway means must contact or pass in close proximity to the oxidation catalyst, and PA1 means for measuring an electrical potential difference between at least two of the following: PA1 a first solid electrolyte having a pair of electrodes located on generally opposite surfaces thereof, PA1 a second solid electrolyte having a pair of electrodes located on generally opposite surfaces thereof, wherein in use a reference gas of known oxygen concentration is supplied to at least one of said electrodes, and a combustion gas is supplied to at least one of said electrodes, and PA1 an equilibrated combustion gas is supplied via a gas pathway means to at least one other of said electrodes, PA1 said gas pathway means arranged such that combustion gas flows therealong and contacts or comes into close proximity with an oxidation catalyst before contacting its respective at least one of said electrodes, PA1 means to measure an electrical potential difference between the pair of electrodes on the first solid electrolyte, PA1 means to measure an electrical potential difference between the pair of electrodes on the second solid electrolyte, PA1 wherein one of said reference gas, combustion gas or equilibrated combustion gas contacts one electrode of each pair of electrodes on the first solid electrolyte and the second solid electrolyte.
Known sensors utilise electrodes formed at the surfaces of the solid electrolyte and measurement of the electrical potential difference (also commonly referred to as `emf` or `voltage`) between the electrodes enables a measurement of the difference between the oxygen partial pressures in the gas streams contacting the electrodes. If a reference gas of known oxygen concentration is used to contact one electrode, the oxygen concentration of a test gas or a combustion gas contacting the other electrode can be determined.
One sensor of known design includes a hollow tube made of a refractory material, such as alumina, and having a disc plug or pellet of solid electrolyte sealed in one end of the tube. Other sensors utilise a tube made entirely of the solid electrolyte. Australian Patent No. 466,251 in the name of Commonwealth Scientific and Industrial Research Organisation, describes various forms of oxygen sensors and the entire contents of this patent are incorporated herein by reference.
Many solid electrolyte materials are known to be suitable for use in oxygen sensors. Examples include zirconia or hafnia, both fully stabilised or partially stabilised by doping with calcia, magnesia, yttria, scandia or one or a number of rare earth oxides and thoria, also doped with calcia, yttria or a suitable rare earth oxide. Australian Patent No. 513,552 discloses the addition of alumina to these solid electrolyte materials to produce a composite solid electrolyte which is particularly suitable for sealing into the end of an alumina tube, thereby making a rugged and leak-tight sensor useful for demanding industrial applications. Australian Patent Application No. 47828/78 disclose the use of magnesium aluminate spinel as an alternative to alumina, either for the supporting tube or as the inert diluent in the composite solid electrolyte material.
The electrodes on solid electrolyte oxygen sensors generally consist of porous coatings of noble metals such as platinum, gold, palladium or silver, or alloys of these elements. For measurements in gases using a gaseous reference an electrode is required on each surface of the solid electrolyte; for measurements in molten metals an electrode is required only on the reference side of the solid electrolyte, and then only if a gaseous reference is used. If a solid reference, e.g., a metal/metal oxide mixture, is used there is no need for a separate noble metal electrode; the solid reference mixture serves as the electrode.
The above described sensors can be used to assist in controlling the fuel/air ratio in combustion processes for efficient fuel use. However, in some instances, the combustion process may not be complete at the point of measurement, so that there may be unused oxygen, unburnt fuel and other products of incomplete combustion present. In this case, the sensors described above may give an accurate but misleading measurement of oxygen concentration, showing what may appear to be excess oxygen but which in fact is oxygen that has not yet been consumed.
It is an object of the present invention to provide a sensor that can detect the presence of non-equilibrium or incomplete combustion in a combustion gas.
In a first aspect, the present invention provides a sensor for detecting non-equilibrium conditions in a combustion gas comprising a solid electrolyte having a first electrode in electrical contact with a first part of the solid electrolyte and a second electrode in electrical contact with a second part of the solid electrolyte, the second part being on an opposite side of the solid electrolyte to the first part, and gas pathway means along which a combustion gas must pass to reach the second electrode, said gas pathway means arranged such that combustion gas passes or contacts an oxidation catalyst before reaching the second electrode.
In use of the sensor of the first aspect of the present invention, the sensor is placed in a combustion gas stream and combustion gas contacts the first electrode. The combustion gas that travels along the gas pathway means must contact the oxidation catalyst before reaching the second electrode. The oxidation catalyst is effective to catalyse the reaction of any unburnt combustible gas with oxygen to form carbon dioxide, and this ensures that the combustion gas that contacts the second electrode has reached equilibrium and that combustion is essentially complete. Oxidation in the presence of the catalyst, also known as "equilibration", consumes excess oxygen present in the combustion gas and accordingly, if the combustion gas stream is not in equilibrium, the second electrode will be exposed to a gas having a lower oxygen partial pressure than the gas stream contacting the first electrode. This will result in an electrical potential difference being produced. Measurement of a potential difference will indicate that the combustion gas stream is not in equilibrium. If, however, the combustion gas stream is in equilibrium, the catalyst in the gas pathway means will not catalyse any oxidation reactions and, the oxygen partial pressure of the gas contacting the second electrode will be the same as the gas contacting the first electrode. In this case, no electrical potential difference will arise.
In a preferred embodiment of the first aspect of the invention, the sensor includes an elongate tube having the solid electrolyte located at and closing one end thereof. The first and second electrodes are located on either of the inner and outer faces of the solid electrolyte, with `inner and outer faces` referring to those faces at the interior and exterior regions of the tube, respectively. Preferably, the first electrode is located on the outer face of the solid electrolyte and the second electrode is located on the inner face of the solid electrolyte. The elongate tube includes one or more apertures therein and, in use, the combustion gas can pass through the one or more apertures and pass along the inside of the elongate tube to contact the second electrode. In this case, the gas pathway means comprises the one or more apertures and an inner bore of the elongate tube. The inner bore of the elongate tube may be coated with an oxidation catalyst or may have an oxidation catalyst arranged therein such that combustion gas entering through the apertures must pass the catalyst or contact the catalyst before reaching the second electrode.
In another embodiment, the elongate tube has an inner rod or tube mounted therein with an annular space being defined between an inner wall of the elongate tube and an outer wall of the inner rod or tube. This annular space defines the gas flow pathway means. The catalyst may comprise a coating of catalyst on the outer wall of the inner rod or tube and/or a coating of catalyst on the inner wall of the elongate tube.
The sensor of the first aspect of the invention should also be provided with means for measuring an electrical potential difference between the first and second electrodes. For example, each electrode may be provided with electrical connections that lead from the respective electrodes and are adapted to be connected to a suitable measuring apparatus. The electrical connectors may comprise conductive paths formed by painting metallic pastes onto the elongate tube and/or the inner rod or tube. Alternatively, the electrical connectors may be one or more wires in electrical contact with each electrode. The skilled addressee will be aware of many arrangements for the electrical connectors that fall within the scope of the present invention and these will not need to be described in greater detail.
The oxidation catalyst may be any catalyst that is capable of catalysing complete combustion. Platinum is the preferred catalyst, although a range of other catalysts, including those used to catalyse automotive exhausts, may also be used.
The electrodes may be of any type suitable for use with solid electrolyte sensors. The electrodes are preferably porous platinum electrodes. The skilled person will recognise that there are many electrodes that will perform satisfactorily and the present invention extends to cover all such electrodes. Electrodes used in solid electrolyte sensors are well known to the skilled addressee and will not be described in greater detail.
In one preferred embodiment of the first aspect of the present invention, the solid electrolyte is described as being located at and closing one end of the elongate tube. This may be achieved by providing a hollow tube, for example of a ceramic or refractory material, and placing a disc, plug or pellet in the end thereof. Alternatively, the elongate tube may be formed from the solid electrolyte and the tube may be made with a closed end.
In another embodiment, the elongate tube may be an open tube and the second electrode located away from the open end and any apertures through which combustion gas may pass. Gas travelling along the tube to the second electrode will still contact the catalyst and this will ensure that the combustion gas has been catalysed and reached equilibrium when it contacts the second electrode.
The present invention also provides a method for detecting non-equilibrium conditions in a combustion gas comprising providing a sensor comprising a solid electrolyte having a first electrode in electrical contact with a first part of the solid electrolyte and a second electrode in electrical contact with a second part of the solid electrolyte, the second part being on an opposite side of the solid electrolyte to the first part, and gas pathway means along which a combustion gas must pass to reach the second electrode, said gas pathway means arranged such that combustion gas passing along said gas pathway means passes or contacts an oxidation catalyst before reaching the second electrode, placing said sensor in a combustion gas stream whereby combustion gas contacts said first electrode and combustion gas passes along said gas pathway means wherein any oxidisable species in said combustion gas are oxidised by the catalyst prior to contacting the second electrode and measuring an electrical potential difference between the first and second electrodes.
In a second aspect, the present invention provides a sensor for measuring oxygen and/or unburnt fuel content of a combustion gas, said sensor including
a) the first and second electrodes; PA2 b) the first and third electrodes, and PA2 c) the second and third electrodes.
Throughout this specification, the term "unburnt fuel" is used to denote the products of incomplete combustion, including raw fuel and carbon monoxide.
Preferably the means for measuring the electrical potential difference includes means for measuring the electrical potential difference between the first and third electrodes and means for measuring the electrical potential difference between the second and third electrodes.
Alternatively, the means for measuring the electrical potential difference includes means for measuring the electrical potential difference between the first and second electrodes and means for measuring the electrical potential difference between the first electrode and the third electrode.
In yet another alternative, the means for measuring the electrical potential difference includes means for measuring the electrical potential difference between the first and second electrodes and means for measuring the electrical potential difference between the second and third electrodes.
In one embodiment of the second aspect of the invention, the sensor includes a first elongate tube having solid electrolyte present at one end thereof, the first electrode being located on an outer face of the solid electrolyte and the second electrode located on an inner surface of the solid electrolyte, a second elongate tube located within the first elongate tube, the second elongate tube having solid electrolyte present at one end thereof, the third electrode being located at an inner surface of the solid electrolyte present at the one end of the second elongate tube, solid electrolyte extending between the second electrode and the third electrode, and the second gas pathway means includes one or more apertures in a side wall of the first elongate tube.
In this embodiment, combustion gas can pass through the one or more apertures in the side wall of the first elongate tube and move through the annular space defined by the outer wall of the second elongate tube and the inner wall of the first elongate tube to reach the second electrode. The second gas pathway means can be seen to include the apertures and the annular space. The oxidation catalyst may be provided in the second gas pathway means by coating one or both of the outer wall of the second elongate tube or the inner wall of the first elongate tube with the oxidation catalyst. This is especially effective in cases where the distance between the inner wall of the first elongate tube and the outer wall of the second elongate tube is small, thereby ensuring that combustion gas travelling along the annular space contacts, or comes into close proximity with, the oxidation catalyst. Alternatively, the catalyst may be provided by locating a gauze of catalyst or particles of catalyst in the annular space or by otherwise locating the catalyst in the annular space.
The means to measure the electrical potential differences may include one or more electrical leads separately connected to each of the first, second and third electrodes. These electrical connections may be similar to those described in respect of the first aspect of the invention and will not be described further.
In use of the sensor of the second aspect of the invention, the sensor is placed in a combustion gas stream. The combustion gas contacts the first electrode. The second electrode is contacted by combustion gas that has been brought into equilibrium by contact with the oxidation catalyst. The third electrode is contacted by reference gas of known oxygen concentration. Due to the differences in oxygen partial pressure of the gas contacting each respective electrode, electrical potential differences are established between the first and second electrodes and the first and third electrodes. Measurement of the electrical potential difference between the first and third electrodes enables the oxygen concentration of the combustion gas to be calculated. Similarly, measurement of the electrical potential difference between the second and third electrodes enables the oxygen concentration of the oxidised (or equilibrated) combustion gas to be measured. The difference in the oxygen concentrations between the raw combustion gas and the equilibrated combustion gas allows the concentration of unburnt fuel in the combustion gas to be determined. The difference in oxygen concentration between the equilibrium and non-equilibrium gas is due to the presence of unburnt fuel. This unburnt fuel consists mainly of carbon monoxide and hydrogen, which react according to the following equation: EQU CO+H.sub.2 +O.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O 3
Equation 3 can be used to calculate the amount of unburnt fuel present in the combustion gas.
The sensor of the second aspect of the invention allows the simultaneous monitoring of both the oxygen and unburnt fuel concentrations of a combustion gas. The sensor provides a unitary device that requires only a single access port in an exhaust duct or other exhaust gas carrying means. The value of the electrical potential difference across the solid electrolyte is dependent upon the temperature of the electrolyte. Therefore, the sensor should incorporate a thermocouple or other temperature measuring means to measure the temperature of the solid electrolyte. Any known thermocouple may be used. A preferred construction includes two wires of dissimilar materials located within the inner tube of the sensor and joined to each other adjacent the solid electrolyte. The two dissimilar wires form the thermocouple, with the hot junction being the above-described junction of the two wires adjacent the solid electrolyte and the cold junction being at a remote location maintained at known temperature.
The sensor of the second aspect of the invention requires only three electrodes. The first and second electrodes are separated by solid electrolyte, as are the second and third electrolytes. The solid electrolyte may constitute a unitary body having the three electrodes formed on appropriate surfaces thereof. Alternatively, the solid electrolyte may comprise two separate pieces of electrolyte, with one piece having two electrodes formed on appropriate surfaces thereof and the other piece having the remaining electrode formed thereon, the other piece of electrolyte being placed in electrical contact with the one piece of electrolyte.
The preferred electrical potential differences measured in the sensor of the second aspect of the invention are the electrical potential differences between the first and third electrodes (which allows the oxygen concentration of the raw combustion gas to be determined) and the second and third electrodes (which allows the oxygen concentration of the equilibrated combustion gas to be determined). This allows oxygen concentrations and the concentration of unburnt combustible gas to be determined.
It is also possible to measure the electrical potential differences between the second and third electrodes, (which enables the oxygen concentration of the equilibrated combustion gas to be determined) and between the first and second electrodes (which allows the difference in oxygen concentration between the raw combustion gas and the equilibrated combustion gas to be determined). Once the oxygen concentration of the raw combustion gas has been determined, the oxygen concentration in the catalysed combustion gas and the concentration of combustible gas in the uncatalysed combustion gas can then be determined.
It is also possible to measure the electrical potential differences between the second and third electrodes, and between the first and second electrodes (which allows the difference in oxygen concentration between the raw combustion gas and the equilibrated combustion gas to be determined). Once the oxygen concentration of the equilibrated combustion gas has been determined, the oxygen concentration of the raw combustion gas and the unburnt fuel concentration of the raw combustion gas can then be determined.
The sensor of the invention may also be constructed such that two pairs of electrodes are used, with each electrode of a pair being separated by solid electrolyte. In this construction, the solid electrolyte of one pair of electrodes does not necessarily have to be in electrical contact with the solid electrolyte of the other pair of electrodes.
Accordingly, in a third aspect, the present invention provides a sensor for determining oxygen and/or unburnt fuel content of a combustion gas comprising
Preferably, the equilibrated combustion gas contacts one electrode of each pair of electrodes.
In a preferred embodiment of the third aspect of the invention, the first solid electrolyte is mounted in or forms part of a first tube and the second solid electrolyte is mounted in or forms part of a second tube, said second tube being located within said first tube. In this embodiment, it is advantageous to supply the reference gas through a bore or passage inside the second tube and to allow the combustion gas to contact the electrode that is located on the outside of the first tube. The annular space located between the inner wall of the first tube and the outer wall of the second tube provides the gas pathway means for supplying equilibrated combustion gas to the inner electrode of the first tube and the outer electrode of the second tube. The annular space is provided with an oxidation catalyst in the same manner as described for the second aspect of the invention.
In all of the aspects of the invention, it is necessary that the gas contacting at least one of the sensors is an equilibrated combustion gas and the invention allows this to be attained by passing the combustion gas along a gas pathway means that includes an oxidation catalyst. The gas pathway must be sufficiently long in relation to its width to ensure that the time of contact between the gas and the catalyst is long enough to cause the gas to essentially reach equilibrium before contacting the relevant electrode.